1. Field of the Invention
The present invention relates to the technical field of wavelength converter and, more particularly, to a wideband wavelength converter based on four-wave-mixing.
2. Description of Related Art
In the development of optical fiber network, time-division multiplexing (TDM) technology is the most popular study issue as the TDM technology is the basic technique used in the circuit-switching of conventional telecommunication. Such a TDM fiber network essentially has two specifications, one of synchronous optical network (SONET) in USA and the other of synchronous digital hierarchy (SDH) in Europe and Japan. Recently, wavelength division multiplexing (WDM) has been applied in optical fiber networks, which can concurrently transmit data of multiple channels (wavelengths) in an optical fiber in a unit of wavelength, thus greatly increasing the bandwidth capability.
In a WDM transmission, a wavelength converter is applied to convert a signal beam into a transmission beam with a certain wavelength for transmission. In optical fiber networks, the WDM wavelength converter needs to meet with high conversion efficiency and wideband wavelength conversion range. At this point, the conversion efficiency indicates that post-converted signal power is divided by ante-converted signal power. Semiconductor optical amplifiers (SOAs) can be applied to configure wavelength converter based on four-wave mixing. When the wavelength converter is converting wavelengths, signal beam and pump beam are injected into an SOA for four-wave mixing, thereby generating a converted signal with the same content as the signal beam. The power ratio of the converted signal over the input signal power is referred to as the conversion efficiency. The conversion efficiency is increased with increasing saturation power and gain of the SOA. However, the conversion efficiency decreases rapidly when wavelength difference of the input signal beam and the converted signal beam is increased.
To overcome the aforementioned problem, in IEEE Photon. Technol. Lett., vol. 10, pp. 952-954, July 1998, a long semiconductor optical amplifier (SOA) is proposed. Since using the longer SOA can provide more optical carriers, the conversion efficiency and the signal to background ratio (SBR) are enhanced. However, a drawback of using the longer SOA is that the induced amplified spontaneous emission (ASE) noise is also enlarged. Besides, using the longer SOA costs higher and the conversion efficiency will degrade rapidly as the detuning range is increasing.
In IEEE Photon. Technol. Lett., vol. 10, pp. 1404-1406, October 1998, a method of using two parallel polarization pumps is proposed, wherein the wavelength difference between the two parallel polarization pumps is fixed. No matter how the gratings are created, the contribution will fall in the same converted wavelength. Thus, the converted signal can be polarized insensitive to the input signal. However, because such a configuration needs two parallel pumps, the conversion efficiency will degrade rapidly as the detuning range is increasing.
In IEEE Photon. Techno. Lett., vol. 11, pp. 982-984, August 1999, there is proposed an “All-optical wavelength translation over 80 nm at 2.5 Gb/s using four-wave mixing in a semiconductor optical amplifier”. In the proposed architecture, two orthogonal-polarization pumps are used to achieve that an 80-nm wavelength conversion range. However, such a semiconductor optical amplifier needs a larger bias current (about 200 mA) that causes a thermal effect, reduces the lifetime of the semiconductor optical amplifier and increases ASE noise. Besides, the conversion efficiency is only about −15 dB and the SBR is only about 22 dB, which are not acceptable. Therefore, it is desirable to provide an improved wavelength conversion to mitigate and/or obviate the aforementioned problems.